Sounds like you are using an IMAP mail account. It also sounds like you are trying to use folder that currently is unknown. Make sure you have actually subscribed to see the 'Trash' folder or whatever the server calls it. eg: I have an imap gmail account and they call it 'Bin'. File > Subscribe select the Trash/Bin folder and click on subscribe. Then make sure your account settings are set up to use it. right click on mail account name in Folder Pane and select 'Settings' select 'Server Settings' for the imap mail account When I delete a message. select: 'Move it to this folder' and select that 'Trash/bin' on imap mail account. click on OK Report back on results.